Why? Has there been debate in favor of gay marriage on FR?
I can't believe I saw none of it if there was.
However, they have been pretty quiet lately with all the courts coming down in favor of banning homo. marriage.
Allow me give you a primer of sorts...
Some of us believe marriage is a sacrament and as such has no business having gov't involvement--leave that to houses of worship.
For gov't interest, primarily to record information in case of divorce or death, any consenting adult couples should be able to participate in civil unions.
This could benefit society by discouraging homosexuals from anonymous and promiscuous behavior (too often in public areas). As such it could help curb health-care costs by decreasing the multiplier effect of STD outbreaks. It benefits society when folks are paired to see one-another into their decline rather than becoming a burden on society. These 'civil unions' should continue to stay a state's right to define.
Since we live in a society which tolerates single-parenthood either never married or surviving spouses, no fault divorce and other libertarian freedoms, you cannot make a strong argument adding 'homosexual marriage' would undermine society/family/etc.
So long as we remained a 'free society', my prediction is a "defense of marriage" codified in the federal constitution would ultimately be repealed within 50 years.